Once a month I fill a plastic egg crate with items I don't need and donate it to the Salvation Army or another local thrift store. It's a good way to get rid of clothes that no longer fit (hopefully due to weight loss) and it helps those in need. Other items are also helpful - the towel with the bleach stain on it can help a shelter animal, the duplicate blanket or coat you got for Christmas can help a homeless person, etc.
If you keep track of it, you can even get a tax write off!
Make sure that the items you are donating are ones that you would be willing to buy new. Charities can't use broken or stained items. Also make sure the charity is taking donations of items before taking them in. Call ahead.